The A type of virus is the most virulent and causes epidemics and pandemics. Where a finer grained classification of the virus strain is needed, this is done by reference to the structural forms of two viral proteins (haemaglutinin and neuraminidase) that are essential to the virus' life cycle. Thus one might speak of H1N1 or H3N2 viral strains.

The virus attacks the respiratory tract, is transmitted from person to person by droplets, and causes the following symptoms:

Although a lot of people in the western world will often call a heavy cold "flu", you know when you have got real influenza as its effects are much more severe and last longer. Typically influenza takes about 1-2 weeks to recover from. Flu can be a killer disease, especially for the weak, old or chronically ill. Some flu pandemics have killed millions of people, for example the "Spanish Flu" pandemic of 1918-1919, which is believed to have killed more people in total than World War I.

Influenza is an extremely variable disease: similar viruses are found in pigs and domestic fowl. In areas where there are high concentrations of humans, pigs and birds in close proximity, such as parts of Asia, simultaneous infections across species enable genetic material to be exchanged between the various strains of flu. This appears to be the principal method by which new infectious strains arise. It is believed that sooner or later, a recombination may occur to produce a strain as lethal as the 1918 virus. In late 1997, a new strain of influenza originating from chickens infected 18 people in Hong Kong, of whom 6 died. This strain did not appear to be readily transmissible from human to human, but such a high mortality rate, and the possibility of a further recombination to make it more infectious, meant that the risk was considered so great that all domestic poultry in Hong Kong was slaughtered. As of April 2002, there have been no further human infections with this particular strain.

Prevention
It is possible to get vaccinated against Influenza, however due to the high mutability of the virus the vaccine is usually only good for a year or so. The World Health Organization co-ordinates the contents of the vaccine each year to contain the most likely strains of the virus to attack the next year. The vaccine is usually recommended for anyone in a high-risk group who may suffer complications from a real flu attack.